The effects of "psychological" stress on the physiology and pathology of the heart will be studied. Modifications in ECG, systemic blood pressure, associated with Sidman avoidance behavior will be measured in squirrel monkeys. Hearts and other organs removed from these animals at the termination of the experiment will be examined for pathological changes. By careful assessment of these data, the changes in physiological parameters induced by "psychological" stress necessary to produce pathological alterations will be ascertained. These investigations will add not only to knowledge about the physiology of cardiac function but also lead to a better understanding of "psychological" factors that elicit cardiac pathology.